Optical communications networks, at one time, were generally “point to point” type networks including a transmitter and a receiver connected by an optical fiber. Such networks are relatively easy to construct but deploy many fibers to connect multiple users. As the number of subscribers connected to the network increases and the fiber count increases rapidly, deploying and managing many fibers becomes complex and expensive.
A passive optical network (PON) addresses this problem by using a single “trunk” fiber from a transmitting end of the network, such as an optical line terminal (OLT), to a remote branching point, which may be up to 20 km or more. One challenge in developing such a PON is utilizing the capacity in the trunk fiber efficiently in order to transmit the maximum possible amount of information on the trunk fiber. Fiber optic communications networks may increase the amount of information carried on a single optical fiber by multiplexing different optical signals on different wavelengths using wavelength division multiplexing (WDM). In a WDM-PON, for example, the single trunk fiber carries optical signals at multiple channel wavelengths to and from the optical branching point and the branching point provides a simple routing function by directing signals of different wavelengths to and from individual subscribers. In this case, each subscriber may be assigned one or more of the channel wavelengths on which to send and/or receive data.
To transmit and receive optical signals over multiple channel wavelengths, the OLT in a WDM-PON may include a multi-channel transmitter optical subassembly (TOSA) and a multi-channel receiver optical subassembly (ROSA). One example of a TOSA includes an array of lasers optically coupled to an arrayed waveguide grating (AWG) to combine multiple optical signals at multiple channel wavelengths. To provide the different channel wavelengths, tunable lasers may be used in the multi-channel TOSA and the wavelengths emitted by the tunable lasers change with changes in temperature. The desired accuracy or precision of the wavelengths in a WDM-PON often depends on the number and spacing of the channel wavelengths and may be controlled in the TOSA by controlling temperature. In a 100 G dense WDM (DWDM) system, for example, the temperature may need to be controlled within ±0.5° C. to maintain a wavelength precision of ±0.5 nm and the temperature range may need to be greater than 4° C. to provide the desired wavelength yield of the laser diodes.
One challenge with such OLT transceiver modules is providing adequate temperature control of the laser array in a relatively small space and with relatively low power consumption. One way to control temperature of the individual laser diodes in a laser array is to use individual temperature control devices, such as thermoelectric (TEC) coolers, and temperature monitors (e.g., thermistors) for each of the respective laser diodes to provide a closed loop temperature control for each of the lasers based on a monitored temperature. To support 16 channels in a TOSA, for example, would require 16 TEC's, 16 thermistors, 32 ports for the thermistors and circuitry for controlling each of these components. Such a closed loop system requires a more complicated circuit design and higher cost and may not fit within a smaller form factor for the OLT transceiver module. Another challenge with heating and temperature control for thermally tuned lasers is operating within a power budget for the OLT module.